The present invention is directed to an electrolytic process and apparatus for recovering copper and other metals. The process and apparatus of the present invention are useful in both electrowinning and electrorefining.
In the present invention a high current density is employed to deposit metal on a cathode. In connection with the foregoing, the term "current density" is the ratio of current in amperes to the area of cathode in square feet and is expressed in ASF units.
The current density normally employed in prior art electrowinning process is about 21 ASF. Of course, it is well known in this art that an increase in current density decreases the time required for a given amount of copper deposition. The main obstacle preventing those skilled in the art from increasing the current density is the lack of a suitable convection system for the electrolyte. The process and apparatus of the present invention is directed to a convection system which permits the current density in an electrolytic deposition process to be increased, while at the same time minimizing the incremental consumption of electrical power. That system includes combination convection baffles and cathode guides on the anodes, or positioned in the tank, a predetermined close cathode-anode spacing, and a gas agitation means positioned below and between the faces of the cathode and anode.
A major benefit to be derived from the application of the method and apparatus of this invention is the elimination of electrical shorts due to contacts between anodes and cathodes. This beneficial feature greatly reduces the amount of "systems work" required in commercial practice to locate and correct electrical shorts.
At the outset it is emphasized that gas agitation in an electrolytic process for recovering metal is not novel. Indeed, there are many prior patents disclosing gas agitation means in deposition systems. The following patents fairly represent the state of the art.
U.S. Pat. No. 1,260,830 to F. E. Studt, entitled Electrolytic Deposition of Copper From Acid Solutions, relates to the electrolytic deposition of copper from acid solutions, wherein a means is provided to continuously agitate the electrolyte, particularly across the face of the anodes. The agitation is provided by a mixture of sulfur dioxide gas and steam. The purpose of the steam is to insure the correct temperature conditions. The pipes through which the mixture of steam and sulfur dioxide is carried contain perforations or nozzles arranged at such angles that the escaping gas and steam will tend to impinge angularly upon the faces of the anodes, so that the electrolyte will be continuously circulated and maximum agitation will occur across the anode faces.
U.S. Pat. No. 1,365,032 to W. E. Greenwalt, entitled Electrolytic Apparatus, uses a mixture of steam and a gas to provide agitation in an electrolyte cell for the deposition of copper.
U.S. Pat. No. 1,365,034 to W. E. Greenwalt, also entitled Electrolytic Apparatus, teaches the use of gas under pressure to agitate the electrolyte in electrolytic copper deposition. Gas under pressure is fed to a perforated hood, and the hood is rapidly rotated in the electrolyte to distribute the gas throughout the bottom of the electrolyte tank.
U.S. Pat. No. 3,412,004 to J. B. Winters, entitled Test Plating Equipment and Method, relates to a laboratory test electroplating apparatus in which an air or compressed gas distribution system is used to cause bubbles to assume random paths of travel through the electrolyte.
U.S. Pat. No. 3,503,856 to R. C. Blackmore, entitled Process for Controlling Electrodeposition, discloses various methods of agitation employed in the electrodeposition of metals.
By and large, however, the prior art methods for agitating the electrolyte have not provided a sufficient amount of convection of the electrolyte which would enable a significant increase in the current density with an attendant production of high quality copper or other metals. Moreover, the prior art agitation methods have not been applicable to electrorefining, because of the resulting suspension of anode slimes and the consequent deterioration of deposit quality.
In the prior art processes which employ non-retentive cathode blanks, the edges of the cathode blanks are masked with non-conducting or insulating material to prevent the metal being deposited on each face of the cathode from joining, which would make removal of the deposit from the cathode difficult. An important side benefit of the convection scheme of the present invention is that insulating edging does not have to be positioned on the edges of a non-retentive cathode in order to prevent the edges of the deposit on each face from joining.
Other important side benefits include guidance of the cathodes into correct position and prevention of electrical contacts between anodes and cathodes, once positioned in the cell.